NATURAL GAS PROCESSING

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NATURAL GAS PROCESSING
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PROPERTIES OF NATURAL GAS

• Density
• Test method ASTM D1070
• Important for storage and transportation
• Compressibility factor for natural gas (Figure
  7.1)
• Gas Hydrates
• Presence of free water, NG-water mixture solidify
  at high P and T above freezing point of H2O
• Hydrates soln of gas in crystalline solids

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PROPERTIES OF NATURAL GAS

• Gas hydrates (continued)
• Correlations predicting gas hydrate formation on
  T-P gradient (Figure 7.2)
• Use antifreeze (methanol, ethylene glycol) for
  lowering freezing points thus prevent hydrates
  formation (Figure 7.3)
• Quantity of methanol depends on total P of gas
  (Figure 7.4)
• Quantity of methanol can be calculated using
  steps in Table 7.9

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TEST

• HC content
• Indirectly by heating value (ASTM D1826) or SG
  (ASTM D1070 or D3588)
• If high amt of CO2 or N2 present in NG, both
  method cannot give a reliable data
• Water content
• Expressed in lb H2O/MCF or Dew point T and P
  (ASTM D1142)
• Two methods have definite relationship (Figure
  7.5)
• Acid gas content
• H2S foul odor, gt 600 ppm is fatal, corrosive to
  all metals 4 ppmv limit
• During combustion sulfur dioxide is produced
  highly toxic, corrosive, poison catalyst

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• Acid gas content (continued)
• CO2 has no heating value
• Removal sometimes necessary to increase energy
  content (BTU/ft3) and prevent solidification in
  cryogenic plants
• Both H2S and CO2 promote hydrate formation (more
  so on CO2)
• Gas liquids
• Define i) ethane ii) LPG iii) natural gasoline
• Ethane max methane 1.5 v/v CO2 0.28 v/v
• LPG compose of propane and/or butane (limits on
  high-MW HC content isopentane content)
• Natural gasoline consists of pentane and
  higher-MW HC
• Other properties
• Heat of combustion, VP, corrosivity and gas
  residue

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Natural Gas Plant General Layout
                                                  
                                                  
                                                  
  
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1. Acid and Gas Removal

• Sour gas
• H2S gt 5.7 mg per m3 NG
• Sweetening NG gas process to remove H2S
• Specs 3 to 4 mole CO2 H2S 4 ppmv
• Processes
• Chemical absorption processes -
  expAlkanolamines, alkaline salt solutions
• Physical adsorption processes Selexol, Rectisol
  
• Pysical chemical process Sulfinol
• Absorption, distillation, adsoprtion, membrane
  and chemical reaction

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• Amine reactions
• Advantages reverse reaction and recover H2S
• Girdle Process (Absorption)
• 95 in US gas sweetening operations
• Sour gas contacted with amine soln (MEA or DEA)
• Amine soln is regenerated
• Solid desiccant process (adsoption)
• Use iron sponges to remove H2S and CO2
• Sulfur
• H2S process to reduce to elemental form S
• Claus process thermal and catalytic reaction

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Amine Process Unit
                                                  
                                                  
                                                  
  
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Sweetening Process Schematic
                                                  
                                                  
                                                  
  
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2. Oil and Condensate Removal

• Wellhead separation
• To process and transport associated NG
• Separate using equipment at or near wellhead
• Process and equipment used vary widely
• Conventional separator
• Closed tank gravity force to separate oil and
  NG
• Low-Temperature separator (LTX)
• Often used for high P gas with light oil or
  condensate
• Pressure differential to cool wet NG and separate
  
• Wet gas ? cooled slightly by heatX ? gas go thru
  high P liquid knockout ? low T (choke
  mechanism) ? gas expand when enters separator ?
  rapid expansion lowers T ? liquid removed ?
  warmed gas thru heatX

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LTX Unit
                                                  
                                                  
                                                  
  
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LTX Schematic
                                                  
                                                  
                                                  
  
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3. Water Removal

• Water in NG
• Most free associated water removed by simple
  extraction method at or near wellhead
• Water vapor in NG solution need more complex
  treatment
• Process of dehydration of NG absorption or
  adsorption
• Pipeline specs 7.0 lb H2O/MMSCF
• Absorption Glycol Dehydration
• Glycol solution (high affinity to water)
  diethylene glycol (DEG) or triethylene Glycol
  (TEG)
• TEG/DEG contact wet gas stream (called contactor)
  ? absorb water ? glycol soln sink to bottom ?
  removed
• Glycol recovery vaporize glycol using special
  boiler
• New tech addition of flash tank separator
  condensers before boiler to condense methane (90
  99 recovery)

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• Solid-Desiccant Dehydration
• Adsorption process consists of 2 or more
  adsoption tower filled with solid desiccant
• At least 1 working, 1 regenerating
• Desiccants activated alumina or granular silica
  gel
• Wet NG ? pass thru towers from top to bottom ?
  H2O retains on particle surface ? dry NG exits ?
  saturated desiccant heated with heater to
  vaporize water
• Best suite for large volumes gas under very high
  P

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Glycol Dehydration Unit
                                                  
                                                  
                                                  
  
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Glycol Dehydrator Schematic
                                                  
                                                  
                                                  
  
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Solid Absorbance Dehydration Unit
                                                  
                                                  
                                                  
  
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Solid Desiccant Dehydration Schematic
                                                  
                                                  
                                                  
  
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4. Separation of Natural Gas Liquids

• Natural Gas Liquids (NGL)
• NG from well contain some NGL
• Higher value if sold as separate products
• Physical chemical process - Sulfinol
• Technique
• Absorption method or cryogenic expander process
  (both 90 of NGL production)
• Two basic steps extract liquid from NG ?
  separate to base components
• Absorption Method
• NGL is absorb by absorbing oil (termed lean
  before absorption and rich oil after
  absorption)
• Fed into oil stills ? heated (Tgtbp of NGL)
• Recovery 75 butane, 85-90 pentane and heavier
  HC

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• Modified absorption using refrigerated oil to
  improve efficiency
• Propane recovery gt90 40 ethane, others 100
• Cryogenic Expansion Process
• Ethane often more difficult to extract from NG
  stream
• Chill gas to -120oF common - turbo-expander
  process
• External refrigerant cool NG stream ? rapidly
  expand chilled gas (expansion turbine) ? T drop ?
  condense ethane and other HC maintain methane in
  gaseous form
• Recovery 90 to 95 ethane
• Energy released from expansion turbine later use
  in extracting ethane process

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Absorption Tower
                                                  
                                                  
                                                  
  
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Absorption Process at Ambient T
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Cryogenic Expander Process
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5. Natural Gas Fractionation

• Process Units
• Deethanizer separates ethane from NGL stream
• Depropanizer separates propane
• Debutanizer boils off butanes leaving pentanes
  and heavier HC in NGL stream
• Butane splitter (Desiobutanizer) separates iso
  and n butanes